Neoplastic transformation of individual cells is believed to be the underlying cause of tumorigenesis. Ionizing radiation is a potent inducer of neoplastic transformation in vitro. Recent studies have shown that both the rate of delivery and the quality of the radiation can markedly affect the observed transformation frequency. Studies with fission-spectrum neutrons at the Janus reactor at the Argonne National Laboratory have indicated that induction of transformation is greatly enhanced when low doses are delivered protracted over time. The implications of these observations could be very important for those occupationally or medicinally exposed. Using rodent cells that have been adapted for the assay of neoplastic transformation in vitro, we propose to determine the following: (1) the dose dependency for neoplastic transformation for a variety of average neutron energies ranging from 0.86 to 25 MeV mean energy; (2) the effect of dose protraction (i.e., low-dose-rate irradiation or multifractionation of high-dose-rate exposures) as a function of average neutron energy of fast neutron beams; (3) the mechanism of the enhanced induction of transformation as can be studied through the use of promoters and antipromoters (including radioprotectors); and (4) the possible basis for the role of antipromoters as a prophylaxis for civilian personnel, medical patients, and military personnel who may be exposed to protracted exposures of neutrons from sources that have a variety of mean energies.